[FIX] sap.m.Popover: Check for classList added in onmousedown
[FIX] sap.m.Popover: Check for classList added in onmousedown

- In IE11, SVG elements don't have a classList,
therefore Popover's onmousedown fails with a runtime error.
Due to this error, further processing of the mousedown
(transformation to touch events) fails as well:
   --> RadioButton does not set the event as marked.
   --> Listitem fires both events.

BCP: 2080128761
Change-Id: I9808b0dccd150c8e177877ca40054daf4315792d

SAP/openui5,SAP/openui5,SAP/openui5,SAP/openui5